Challenges and opportunities of ZnO-related single crystalline heterostructures Y. Kozuka1, A. Tsukazaki2,3, M. Kawasaki1,4,a) 1Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan 2Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 3PRESTO, Japan Science and Technology Agency (JST), Tokyo 102-0075, Japan 4 RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan Abstract Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy (MBE) and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with MgxZn1-xO. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make MgxZn1-xO as intrinsic as possible has opened the possibilities of making p-type MgxZn1-xO through NH3 doping method. For the latter, conducting and transparent polymer films spin-coated on MgxZn1-xO was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the MgxZn1-xO/ZnO interfaces are promising candidate for quantum devices because of high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance. a)Electronic mail: [email protected] 1 TABLE OF CONTENTS I. INTRODUCTION ................................................................................................................ 3 II. HOMOEPITAXIAL GROWTH ......................................................................................... 4 A. Reduction of impurities in ZnO substrate........................................................................ 4 B. Growth conditions of ZnO films ...................................................................................... 6 III. GRWOTH OF MGxZN1-xO FILMS ................................................................................... 6 A. Determination of Mg content ........................................................................................... 6 B. Mg dependence of physical parameter of MgxZn1-xO ....................................................... 8 IV. SCHOTTKY CONTACT ................................................................................................... 9 A. PEDOT:PSS/ZnO junction .............................................................................................. 9 B. Schottky photodetector ................................................................................................... 11 V. LIGHT EMITTER ............................................................................................................ 12 VI. TWO-DIMENSIONAL ELECTRON GAS ...................................................................... 13 A. Formation mechanism ................................................................................................... 13 B. Depth profile of the 2DEG ............................................................................................. 13 C. 2DEG density tuning ..................................................................................................... 14 E. Evolution of quantum Hall effect ................................................................................... 16 F. Correlated 2DEG in MgxZn1-xO/ZnO ............................................................................. 17 G. Spin coherence time ....................................................................................................... 19 VII. CONCLUSIONS ............................................................................................................ 20 2 I. INTRODUCTION The aim of this review article is to overview the rapid progress of thin film techniques to grow ZnO based heterostructures on ZnO single crystalline substrates and to introduce novel physical properties and device functionalities that have become possible only for those heterostructures rather than conventional thin films of the same materials. There has been a long history of the research on Zinc Oxide (ZnO) as summarized in Fig. 1. Bulk form of ZnO such as ceramics and powders have been quite useful as chemical ingredients in rubbers, glasses, catalysts, pharmaceutical products like sunscreen, as well as magnetic ferrites and varistors as surge arrestor.1 Basic properties of ZnO are summarized as follows: 1) a wide direct band gap of 3.37 eV, 2) a large exciton biding energy of 60 meV, 3) large spontaneous polarization and piezoelectric constants, 4) high purity, and 5) popular price and harmless as some of them are listed in Table I. For the electronics applications as passive components, most intensively studied topics are transparent conducting oxide films to be used for photovoltaics and displays.2,3 In the last decade or two, thin films studies have been rather focused on active device elements such as ultraviolet (UV) emitters and photodetectors as well as transparent transistors, with utilizing semiconducting features and transparency. Various kinds of reviews have been already reported, which highlight, for example, varistor application,1 surface chemistry and physics,4 defects,5,6 ferromagnetism,6,7 nanostructures,6,8,9 Schottky junction,10 UV emittion,6,11 and transparent field-effect transistor.2,3 One of the stimulating results triggering intensive studies aiming at such active device elements is the observation of highly efficient UV laser emission from thin films by photo excitation.12,13 This experiments clearly indicated that ZnO thin films can be reached to the level of active semiconductor grade. For further exploring the possibility, it was evident that pursuing better thin film technique is essential to fabricate heterostructures as designed for active devices. Although there have been numbers of studies in this stream, we concentrate in this review article on the thin film techniques and devices on single crystalline ZnO substrates which are highlighted in blue characters in Fig. 1. The use of single crystalline substrates and molecular beam epitaxy (MBE) has drastically improved the performance of such devices as UV emitter, UV photodetector, and transistors. Especially two-dimensional (2D) electron transport with extremely high mobility has opened up a new research field on the quantum transport phenomena and possible quantum devices in oxides. Many of the interesting functionalities originate from the Wurtzite crystal structure of ZnO as shown in Fig. 2. In this crystal structure, Zn is shifted from the 3 center of octahedron with oxygen at vertices and the crystal does not possess inversion symmetry, showing a large spontaneous polarization along [0001] crystalline direction. _ Most studies on thin films have been focused on the Zn-polar (0001) or O-polar (0001) due to energetic stability of the surfaces compared to other planes. Actually, thin films presented in this review article were mostly grown on Zn-plane ZnO substrate by molecular-beam epitaxy (MBE) due to the chemical stability against acid, high nitrogen doping efficiency, and proper direction of electrical polarization for creating 2D electron system. In Sections II and III, we introduce the growth optimization of ZnO and MgxZn1-xO thin films, respectively, on well-treated Zn-polar ZnO single crystal substrates. Then, we will discuss the various electrical or optical properties for the heteroepitaxial junctions. In Section IV, we describe photodetector comprised of conducting polymer/ZnO Schottky junction, which provides high quantum efficiency probably due to an abrupt and clean interface of organic-oxide constituents. In Section V, UV light emitter is overviewed based on p-type MgxZn1-xO / n-type ZnO junctions. In Section VI, 2D electron system confined at the MgxZn1-xO/ZnO heterointerface is introduced as a potential candidate for the channel of quantum devices utilizing high electron mobility of 800,000 cm2 V-1 s-1. Finally in Section VII, summary and future prospects are given for ZnO devices. II. HOMOEPITAXIAL GROWTH A. Reduction of impurities in ZnO substrate Highly pure ZnO single crystals have been grown by vapor transport and hydrothermal methods.14-16 In early stage of ZnO studies from 1960’s to 90’s, excitonic features had been intensively investigated by optical measurements of high quality single crystals.17-21 In addition, surface charge transport had also been studied based on single crystal surfaces with electrolyte or ion implantations of